Keynes developed an elegant theory; easy to understand; and readily adopted by the political classes. It was an easy sell to the public, because it seemed like a short cut to easy street.
It was doomed to failure from the getgo. The reason was not found in convoluted economic theory and arguments. The reason is simple and irrefutable. It provides political cover for deficit spending...deficits now vs. surpluses later.
Deficit spending, of course, became an easy addiction. Like all drug habits, it consumed everything in sight.